The efficiency of a Radio Frequency (RF) Power Amplifier (PA) is generally defined as a ratio between a desirable transmitted radio power and a total power from a power supply. However, this ratio appears to be rather low in emerging wideband applications and beyond if traditional architectures are still employed. For a purpose of enhancing the efficiency, an Envelope Tracking (ET) technique has been proposed and utilized in the wireless communication industry, particularly in user equipment and base stations. The ET technique refers to an approach to RF amplifier design, in which the power supply voltage applied to the power amplifier is continuously adjusted to ensure that the amplifier is operating at peak efficiency for power required at each instant of transmission. Moreover, the ET technique has been considered as a most promising efficiency enhancement solution for the fourth Generation (4G) and beyond wireless communications.
Currently, Orthogonal Frequency Division Multiplexing (OFDM) is widely regarded as the key underlying air interface technology for wireless systems such as WiMAX, the third Generation Partnership Project (3GPP) Long Term Evolution (LTE) and 3GPP2 Ultra Mobile Broadband (UMB), Digital Video Broadcasting (DVB), and Wireless Local Area Network (WLAN). Due to the inherent digital modulation nature of these technologies, OFDM signals have high Peak-to Average Power Ratio (PAPR) that adversely impacts the efficiency of PAs used in wireless at both base stations and user equipment.
If it applies envelope tracking in OFDM system to enhance ET PA system performance, the challenges of the ET PA lies in that the performance which is good for 3G CDMA systems may exhibit poor performance when used in conjunction with OFDM signals, given the stringent Error Vector Magnitude (EVM) and spectral emission requirements specified in 3GPP standard because of high PAPR signal characteristics.
Besides, it is known that in the ET PA, the power supply voltage applied to the PA is constantly adjusted in accordance with the envelope version of an original input signal to ensure that the PA is operating at the peak efficiency over the output power range. However, when the PA power supply is changed from low to high instantaneously and dynamically or vice-versa, the PA operating condition on the drain-side would change dramatically accordingly. This significant change of the PA operating point would give rise to undesirable distortions and memory effects, which may cause gain collapse and unpredictable and non-correctable distortions and adversely affect both efficiency and linearity of an ET PA system.